This invention relates to optically active film composites, and in particular but not exclusively to window film of the type adhered to the surfaces of already existing windows of buildings and vehicles.
Certain block copolymer solutions are known to form distinctive morphologies and may spontaneously separate into microdomains when they are cooled from the melt or solvent is evaporated from solutions of the copolymers. The size, shape, spacing and number of the microdomains can be controlled through the selection of the relative amounts of the various comonomers, their molecular weight, and the thermodynamic incompatibility of the copolymer components. All this is disclosed in U.S. Pat. No. 5656205 and U.S. Pat. No. 5622668. In particular it is known that when the percentage volumes of the two components of the copolymers are substantially 50:50 the block copolymer will segregate into lamellar microdomains of the component polymers.
French Patent 2138 645 discloses a method for making high molecular weight block copolymers having lamellar layers of different refractive indices.
The present invention uses the known properties of block copolymers to produce film composites which are particularly useful for window film.
Accordingly there is provided an optically active layered composite comprising a substrate having thereon at least one layer of block copolymer film comprising at least two polymeric components separated into lamellar microdomains of each polymeric component, which components have different refractive indices.
Optical properties include reflectance properties and light transmission properties.
Typically the block copolymer comprises between 30:70 and 70:30 volume percent of each component of the copolymer, preferably between 40:60 to 60:40 volume percent of each component, and more preferably 50:50 by volume of each component.
Polystyrene:polybutadiene (PS-PB) block copolymers can produce lamellar structures with a styrene butadiene volume ratio of 30:70, and polybutadiene-polyethylene oxide block copolymers will produce lamellar structures with a 30:70 volume ratio of butadiene:ethylene oxide.
The block copolymer film is optically clear, that is essentially haze free, and the difference between the refractive indices of the two components should be at least 0.08. The block copolymer layer preferably reflects at least 50% of incident light of selected wavelengths.
The substrate may be covered in a plurality of layers of block copolymer film, and the repective block copolymer in one layer may be different than the respective copolymer in another layer so that different film layers selectively reflect different wavelegths of incident light.
The preferred substrate is a transparent substrate, preferably a polymeric film, typically polyethylene terephthalate (PET), and preferably the block copolymer layer is sandwiched between two film substrates. The substrate film may have a thickness of about 12-50 microns, as is typically used for window film.
The block copolymer typically contains as one component a high refractive index polymer such as polystyrene and the other component may comprise a lower refractive index polymer such as at least one of the following: polyisoprene, polybutadiene, polymethyl methacrylate, polydimethylsiloxane, polyethylene-butylene (hydrogenated polybutadiene).
The copolymer may be in form of diblock copolymers A-B where A and B are different polymer components, or triblock copolymers A-B-A.
The lamellar microdomains are formed with the two polymers forming alternating domains of components A and B formed from an AB or ABA block copolymer. The thickness of a pair of adjacent domains in lamellar morphology is referred to as the xe2x80x9cdxe2x80x9d spacing. The xe2x80x9cdxe2x80x9d spacing is determined by the molecular weight (MW) of the copolymer and will be the same (for a given MW) for AB or ABA copolymer.
The thickness of the lamellae is related to the molecular weight by the equation:
d=K Mn⅔
where k is a constant for the particular pair of polymers in the block copolymer; and
d is the lamellar thickness of two adjacent domains in nanometer; and
Mn=number average molecular weight of the copolymer in g/mole.
For example, for a styrene:butadiene block copolymer
d=0.024 Mn⅔
(Hashimoto et al Macromolecules 1980, 13, 1237)
It is therefore possible to make film composites that reflect light of a particular band width by selection of the molecular weight of the copolymer.
For a PS-PB block copolymer the number average molecular weight (Mn) of the block copolymer is in the range of 200,000 to 2000,000, preferably 250,000-1000000, and more preferably 300,000-600,000.
The xe2x80x9cdxe2x80x9d spacing is not affected in theory by the thickness of the applied block copolymer layer. Thickness does affect the number of lamellar domain pairs. For example, a 1 micron coating of an AB block copolymer having a molecular weight such that it forms a lamellar morphology with a xe2x80x9cdxe2x80x9d spacing of 100 nm, will segregate into 10 lamellar domain pairs of A and B, and hence 20 alternating lamellae of A and B.
Reflection is best provided by lamellar domains having a thickness of about xc2xc wavelength. The wave length of light varies with the refractive index of the material through which it is passing according to the formula:       λ    ⁢          xe2x80x83        ⁢    material    =            λ      ⁢              xe2x80x83            ⁢      air              η      ⁢              xe2x80x83            ⁢      material      
where xcex=wavelength, xcex7= refractive index.
The film composite may be made to reflect particular wave bands of light by selection of the copolymer component to provide xe2x80x9ctailoredxe2x80x9d average thickness lamellae. For example for a block copolymer having a refractive index of between 1.5-1.6 (typical for polymers) a UV light (xcex=350 nm) is reflected by lamellae having a thickness of about 60 nm. IR light (xcex800-1500 nm) is reflected by lamellae having a thickness of between 140-250 nm, and visible light (xcex400-800 nm) is reflected by lamellae having a thickness of between 70-140 nm. In practice, it is believed that the maximum thickness of lamellae that can produced will be about 170 nm.
The xe2x80x9cdxe2x80x9d spacing may also be increased by the inclusion of a diluent either in the form of a compatible solvent, plasticizer, or homopolymer. Typical solvents are cumene, or chloroform with PS-PB block copolymer, or toluene with the block copolymers of polystyrene/polyisoprene; polystyrene/polybutadiene; polystyrene/polymethyl methacrylate; polystyrene/polydimethylsiloxane. Typical plasticizers may include hydrocarbon oils for use with polybutadiene.
The volume fractions of each copolymer may be made up entirely of the respective copolymer component, or copolymer plus a compatible homopolymer diluent. The compatible homopolymer may comprise the homopolymer of a respective copolymer component. For example polystyrene or polybutadiene may be mixed with PS-PB block copolymer, to achieve a total volume ratio of polystyrene:polybutadiene of about 1:1. Alternatively, or additionally the homopolymer may comprise a different homopolymer, for example poly 2,6 dimethylphenylene oxide may be added to swell the polystyrene phase of a block copolymer.
Homopolymer polystyrene may be added to a PS-PB copolymer consisting of a 30:70 ratio polystyrene:polybutadiene to raise the polystyrene ratio from 30:70 to 60:40 polystyrene:polybutadiene. The diluent homopolymer should have a lower molecular weight than the block copolymer to which it is added. Preferably the molecular weight of the homopolymer should not exceed 40% of the molecular weight of the block copolymer to which it is added.
When a block copolymer is blended with a homopolymer the properties of the lamellar domains are related to the molecular weights of the-various components by a factor xcex1, where:   α  =                              (          NaNb          )                    Nab         less than     0.5  
where Na is the number average degree of polymerisation of homopolymer component a, Nb is the number average degree of polymerisation of homopolymer component b, Nab is the number average degree of polymerisation of the diblock copolymer.
The smaller is a then the flatter are the lamellar domains and the interfaces between adjacent lamellar domains. Preferably xcex1 less than 0.2, and for best light transmission and reflection properties xcex1 less than 0.1.
The volume fraction of homopolymer which can be added to the block copolymer to form a blend is also related to xcex1 such that the smaller is xcex1 then the more homopolymer can be added to the blend. The addition of homopolymer makes the blend less viscous and easier to process as well a producing a cheaper blend.
Preferably, the maximum volume fraction of homopolymer in the blend should not exceed 0.8.
The xe2x80x9cdxe2x80x9d spacing for the lamellar domains is also dependant upon the volume fraction of homopolymer xcfx86h in the blend such that there is correlation between the xe2x80x9cdxe2x80x9d spacing of the pure block copolymer Do and the xe2x80x9cdxe2x80x9d spacing of the blend Db, such that Db increases with increasing volume fraction of the homopolymer up to a maximum where Do:Db is between 1:4 and 1:5. For a PS-PB block copolymer, either or both homopolymers may be used dependant upon which lamellar domains require swelling to achieve particular optical properties.
The copolymer blend may include antioxidants, heat and light stabilizers, and UV absorbers as additives. Optically active additives may be added, for example dye, and/or particles of a high refractive index material which are preferably added to the higher refractive index component of the copolymer blend. High refractive index particles include antimony tin oxide, indium tin oxide, titanium dioxide, and iron oxide.
Also according to the present invention, there is provided a light reflective film composite comprising a transparent substrate coated by block copolymer comprising two polymeric components separated into lamellar microdomains, which components have different refractive indices, and at least one of the components including particles of material having a different refractive index from said one component. Preferably said one component is the higher refractive index component, and the particles are particles of a high refractive index material.
One method for making block copolymer film coatings is by spin casting the block copolymer in a solvent, typically toluene, the solutions being coated onto a substrate with the evaporation of the solvent and microphase separation taking place gradually from the exposed surface adjacent the air interface and inwardly as the solution evaporates.
The thickness of the coating may be controlled by a draw bar. Thin coatings less than 250 xcexcm encourage the lamellae to form substantially parallel to the substrate.
Preferred methods of coating a block copolymer solution are by means of roller coating especially by the use of gravure coating and printing techniques and slot die coating.
Also, according to the invention, there is provided a method of making of an optically active composite film in which method a solution of block copolymer blend is coated onto a film substrate and is then caused to separate into lamellar microdomains of each polymeric components, which components have different refractive indices.
Preferably the optically active film can be made to reflect a desired percentage of light and/or a desired wave band of light by controlling the difference in refractive indices, the thickness of the lamellar microdomains, and/or their number.
The thickness of the microdomains may be controlled by the addition of diluents for example homopolymers having a lower molecular weight than the polymer fractions to which they are added.
Film composite according to the present invention may be applied to vehicle windows, the windows of building, in particular the windows of pre-existing buildings, PC monitor screens, video screens, and any other surface which may be required to be optically active for example packaging, show cases and display stands.